FIG. 1 illustrates the frame structure of a multi-carrier mode in a wireless LAN, which includes a preamble sequence and data to be received. Generally, the multi-carrier mode in the wireless LAN employs the preamble sequence to achieve accurate synchronization and channel estimation. The preamble sequence includes two parts, a short training sequence and a long training sequence, wherein the short training sequence is consisted of ten duplicate short training symbols (A0, A1 . . . A9), and each short training symbol contains 16 sample points; the long training sequence includes a 32-sample CP (Cyclic Prefix) and two duplicate long training symbols (L1 and L2), each long training symbol contains 64 sample points. In general, a receiver employs the short training sequence to perform timing synchronization and carrier frequency offset acquisition, and employs the long training sequence to accurately estimate the timing position and the carrier frequency offset and to provide an accurate channel estimation.
The initial wireless LAN standard provided a bandwidth of 20 MHz for the multi-carrier mode. Then, two transmission modes were proposed to meet the requirements of application, one is “half-clocked” mode corresponding to transmission bandwidths of 10 MHz, another is “quarter-clocked” mode corresponding to transmission bandwidths of 5 MHz. It is easily found from analysis that the reduction of transmission bandwidth is accompanied with the reduction of subcarrier spacing and the increase of sampling period, as indicated by the following Table 1.
TABLE 1Transmission ModeSystem Parameter20 MHz10 MHz5 MHzNumber of FFT Point646464Subcarrier Spacing312.5 KHz156.25 KHz78.125 KHzEffective Bandwidth 16.6 MHz  8.3 MHz 4.15 MHz
It should be noted that the reduction of transmission bandwidth has a direct impact on the acquisition range of carrier frequency offset. Common carrier frequency offset acquisition methods can achieve the acquisition of a maximum carrier frequency offset of 625 KHz in the 20 MHz transmission mode, 312.5 KHz in the 10 MHz mode, and 156.25 KHz in the 5 MHz mode. And when a carrier frequency offset in the actual system exceeds the maximum acquisition range, the receiver can not correctly receive data.
To ensure that the data will not be affected by large carrier frequency offset during the reception phase, the carrier frequency acquisition should be accomplished during the reception of the short training sequence. Therefore, the research on dealing with carrier frequency offset within a large range by directly using the received short training sequence is of great practical significance.